The role of salps in marine ecosystem function

Off southeast Australia during spring, the time of maximum productivity, the biomass of grazers on phytoplankton is often dominated by salps. Salps are a large fast-growing gelatinous zooplankton that consume phytoplankton which are many orders of magnitude smaller than themselves. It has been suggested that these characteristics give salp populations the potential to alter regional food-webs and even global fluxes of carbon (Andersen, 1998).

Early researchers viewed the marine food web as being composed of large phytoplankton (diatoms) being grazed by copepods, which are consumed by larval fish, and finally by large predators (Nybakken, 2005). In the 1980s, the importance of small phytoplankton, microzooplankton and bacteria in the recycling of nutrients was established, and coined the microbial loop (Azam et al., 1983). Recently, the importance of gelatinous zooplankton such as salps and larvaceans in modifying the marine food web has been speculated (Andersen, 1998). Due to the large difference between their own size and the phytoplankton they consume, salps are not easily incorporated into either the classical food web or microbial loop paradigms.

An exhaustive, 2.5 year survey of zooplankton off southeast Australia from May 1938 to January 1941 (Thompson, 1948), involving 304 stations, established that tunicates (salps and a closely related class larvaceans) are the second most abundant class of zooplankton (the first being copepods). Dense salp swarms have often been observed off Sydney (Heron and Benham, 1984) and shown to drastically reduced phytoplankton abundance (Humprey, 1963). On a recent research cruise off southeast Australia in September 2006 (RV Southern Surveyor - SS0906) salps were the dominant plankton collected in both 1 mm midwater and 0.5 mm surface nets. In many cases, especially in the vicinity of Lord Howe Island, the biovolume of salps was an order of magnitude greater than all other class of zooplankton combined. In spring, at the time of maximum primary productivity, and a time of critical importance for the structuring of marine ecosystems and carbon fluxes, salp blooms are a regular phenomenon off southeast Australia.

Gelatinous zooplankton, and in particular salps, have ecological attributes that separate them from non-gelatinous zooplankton such as copepods. The most common salp of southeast Australia, Thalia democratica, can reproduce both sexually and asexually, with an individual or solitary form capable of producing a chain of individuals (Miller, 1997). This extremely effective reproductive strategy can result in population increases of up to 2.5 times per day (Heron, 1972b). While such a population growth rate is common in protozoans (single cell animals), it is exceptional among metazoans (multi-cellular animals). Salps also consume phytoplankton and bacteria that are at the bottom of the food chain (Muller, 1983), allowing them to respond quickly to increases in primary production. Salps are often either avoided or a prey of last resort for predators such as fish (Mianzan et al., 2001), due to their low nutritional value.

This set of ecological attributes has led researchers to investigate regionally and globally important effects of salps (Anderson, 1998). Salps compete with other zooplankton such as copepods. In the Southern Ocean, for example, a decrease in krill populations over the last 50 years has been accompanied by an increase in salp populations (Atkinson et al., 2004). In sub-tropical waters, the relative abundance of salps in the zooplankton community will alter the balance between those predator species that avoid salps, and those species such as warty oreo (Lyle and Smith, 1997) for whom salps are an important component of their diet. The vertical flux of carbon due to salps is considered to be potentially greater than that of non-gelatinous zooplankton. Salps consume a large prey range, including the smallest phytoplankton (Mullin, 1983), resulting in efficient movement of biomass from small to large particulate matter. Salps are relatively large, and therefore, for same density difference to the surrounding water, will sink faster than smaller components of the zooplankton. By forming chains, this effect is compounded. The fecal pellets that salps generate have been estimated to sink at up to 1000 m d-1, an order of magnitude faster than copepod fecal pellets (Walsh, 2001) Furthermore, salps often vertically migrate, and commonly are consumed by benthic predators (Lyle and Smith, 1997), directly delivering carbon to the seafloor.

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